X-ray intensifying screens are generally used in conjunction with silver halide photographic films and serve to enhance the image formed on that film. Phosphors, which are the active component of X-ray screens, are legion in number and include the tungstates, the oxysulfides, and the oxybromides, among others.
Particularly efficient phosphors which may be used in the preparation of an X-ray intensifying screen are the tantalates described by Brixner in U.S. Pat. No. 4,225,623. These phosphors are based on yttrium, lutetium, and gadolinium tantalates of the M' monoclinic form and are generally activated with rare earths such as terbium, thulium and niobium, for example, as well described in the aforementioned reference. Since these phosphors have a high X-ray stopping power, they are presently widely used for the preparation of these intensifying screens and the method for their preparation includes the mixing of ingredients followed by firing this mixture to form the phosphor crystal lattice itself.
During this firing step it is often beneficial to use a flux which usually forms a partial liquid at the elevated temperatures commonly used. Thus, the flux can be thought of as a fluid in which the various component parts of the phosphor react to form the phosphor. These fluxes are generally alkali metal salts which are then removed from the reaction mixture by washing. Commonly fluxes include: Li.sub.2 SO.sub.4, pure LiCl, BaCl.sub.2, SrCl.sub.2, known and mixtures of two of these salts, for example. Some fluxes are better to use than others since at the elevated temperature in which the phosphor is fired, some of the halide salts will vaporize and corrode the equipment. Lithium sulfate may decompose at these temperatures to give lithium oxide and oxides of sulfur. Lithium oxide is very reactive and can form lithium tantalate and/or lithium niobate in the phosphor oxide mixture. Lithium tantalate and niobate, however, are not as efficient as, for example, the yttrium compounds when used as X-ray phosphors. Thus, there is a need to find better flux systems for the preparation of X-ray intensifying phosphors.
The use of a rare earth oxide phosphor containing alkali metal silicates and germanates, is also known. In this particular case, however, the silicate is used as an integral mixture with the phosphor itself and it is reported that the brightness of the rare earth oxide phosphor is increased. The phosphors produced by this technique are not X-ray intensifying phosphors, but are red-emitting phosphors used in cathode ray tubes, for example.
It is an object of this invention to provide a tantalate phosphor of the M' monoclinic form with improved phosphor efficiency and purity which are achieved by reducing the level of contaminants in the phosphor reaction mixture.